The present invention relates to a procedure for manufacturing cellulose carbamate by reacting isocyanic acid and cellulose at elevated temperature.
Both Finnish Pat. Nos. 61033 and 62318 disclose a procedure for manufacturing an alkali-soluble cellulose derivative from cellulose and urea. The procedure is based on the fact that when urea is heated to its melting point or to a higher temperature it begins to decompose to isocyanic acid and ammonia. The isocyanic acid reacts with cellulose, producing an alkali-soluble cellulose derivative, called cellulose carbamate. The reaction may be written as follows: ##STR1## The cellulose compound thus produced, namely cellulose carbamate, may be dried after washing and stored even for prolonged periods, or it may be dissolved, for instance for fiber production, in an aqueous alkali solution. Cellulose carbamate fibers or films may be manufactured from such solution by spinning or by extruding, in the same manner as in the viscose manufacturing process. The stability of cellulose carbamate and the feasibility of its transport in dry state afford a great advantage compared with cellulose xanthate in the viscose method, since the latter cannot be stored or transported, neither in dry nor in solution form.
When urea decomposes under the effect of heat to isocyanic acid and ammonia, certain additional side reactions occur. One such side reaction is the reaction of isocyanic acid with urea, whereby biuret is produced. This biuret is still present, after the carbamate reaction, in the carbamate that has been produced and has to be removed, e.g. by washing. Moreover, it is necessary to remove, from the carbamate, the urea that has not reacted or has not been decomposed, the amount of which may be considerable at the start.
Efforts are naturally made in any industrial process to recover chamicals that are required in the reaction and to recycle the same for reuse in the process. In a continuous carbamate process, this involved separation of urea from the cellulose carbamate and, furthermore, separation from urea of the biuret produced as a by-product. The removal step increases the costs of the process.